1. Field of the Invention
The present invention relates to measurement of overlay usable, for example, in the manufacture of devices by lithographic techniques. Specifically, the present invention relates to measuring targets comprising a first marker and a second marker and each target having a different overlay bias between its first and second marker and using the measurements to determine an overlay result.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
The 32nm half pitch (HP) node and beyond will require double patterning immersion lithography (while Extreme Ultra Violet lithography is not ready for mass production). Various known process schemes exist for achieving 32nm HP, amongst them are Litho Freeze Litho Etch (LFLE) and Litho Etch Litho Etch (LELE).
In order to establish acceptable overlay control for control between the two layers, overlay can be measured in various ways. However, overlay is typically not measured at the printed resolution because its measurement is not straightforward. Usually overlay is measured on specifically designed macro-pitch targets and measured using image based or scatterometry based methods. These special features usually do not resemble the product or product pitch. Therefore the link with the real at-resolution overlay is lost. Furthermore, scatterometry methods cannot reconstruct the dual line pattern of double patterning structures at resolution due to loss of sensitivity near the symmetry point.
Overlay at-resolution is sometimes measured on resolution targets using a CD (Critical Dimensions) SEM (Scanning Electron Microscope). That method is expensive, takes time and is not accurate enough (estimated at ˜1-2 nm).